I merge 2 vertex and become weird ,How can i fix?
To me, it looks like you have not cleared one side of your mesh before setting up the mirror modifier.
So if you still have both sides of the mesh and you're trying to merge things from both sides, the result is quite straightforward.

The mirror modifier is meant to literally duplicate symmetrically your mesh. Its purpose is for you to handle only a fraction of the mesh. But you have to cut out the part you want to let to the mirror modifier first.

So, if that's the issue: 

In edit mode, Numpad 1 for front view, use Z to set the wireframe mode (it will allow you to select hidden geometry).
Select one side of your mesh up to the center with B and delete it.